1. Field of the Invention
The present invention generally relates to the fabrication of high-quality, quantum dot-based films. More particularly, but not exclusively, it relates to the fabrication of high-quality quantum dot polymer films using a chain transfer agent as an additive during manufacturing.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
There has been substantial interest in exploiting compound semiconductors having particle dimensions on the order of 2-50 nm, often referred to as quantum dots (QDs), nanoparticles, and/or nanocrystals. These materials have high commercial interest due to their size-tunable electronic properties, which can be exploited in a broad range of commercial applications. Such applications include optical and electronic devices, biological labeling, photovoltaics, catalysis, biological imaging, light emitting diodes (LEDs), general space lighting, and electroluminescent displays.
Well-known QDs are nanoparticles comprising metal chalcogenides (e.g, CdSe or ZnS). Less-studied nanoparticles include III-V materials, such as InP, and including compositionally graded and alloyed dots. QDs typically range from 2 to 10 nanometers in diameter (about the width of 50 atoms), but may be larger, for example up to about 100 nanometers. Because of their small size, quantum dots display unique optical and electrical properties that are different in character to those of the corresponding bulk material. The most immediately apparent optical property is the emission of photons under excitation. The wavelength of these photon emissions depends on the size of the quantum dot.
The ability to precisely control quantum dot size enables a manufacturer to determine the wavelength of its emission, which in turn determines the color of light the human eye perceives. Quantum dots may therefore be “tuned” during production to emit light of a desired color. The ability to control or “tune” the emission from the quantum dot by changing its core size is called the “size quantization effect.” The smaller the QD, the higher the energy, i.e., the more “blue” its emission is. Likewise, larger QDs emit light more toward the electromagnetic spectrum's red end. Dots may even be tuned beyond visible light into the spectrum's infra-red or ultra-violet band. Once synthesized, quantum dots are either in powder or solution form. Because of their tiny size, the ability to produce even a relatively “small” volume of quantum dots (e.g., one kilo) will yield enough actual quantum dots for industrial scale applications.
A particularly attractive application for quantum dots is in the development of next generation LEDs. LEDs are becoming increasingly important in modern day life, and it's predicted that they have the potential to become a major target for quantum dot applications. Quantum dots can enhance LEDs in a number of areas, including automobile lighting, traffic signals, general lighting, liquid crystal display (LCD) backlight units (BLUs), and display screens. At present, LED devices are made from inorganic solid-state compound semiconductors, such as GaN (blue), AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue). Unfortunately, the technology does not yet exist to produce solid-state LEDs that emit white light. One solution is to combine solid-state LEDs of different frequencies to produce a white light; however, it's difficult and expensive to produce “pure” colors. Alternatively, solid-state LEDs can be down-converted to white light by placing a combination of phosphor materials on top of the LEDs. The light from the LED (the “primary light”) is absorbed by the phosphor material and re-emitted at a second frequency (the “secondary light”), which produces a white light. Down-converted LEDs cost less and are simpler to fabricate than LED combinations; however, conventional phosphor technology produces light with poor color rendering (i.e. a color rendering index (CRI)<75).
Quantum dots are a promising alternative to conventional phosphor technology. First, their emission wavelength can be tuned by manipulating nanoparticle size. Second, so long as the quantum dots are monodispersed, they exhibit strong absorption properties, narrow emission bandwidth, and low scattering. Rudimentary quantum dot-based light-emitting devices have been manufactured by embedding colloidally produced quantum dots in an optically transparent (or sufficiently transparent) LED encapsulation film, such as silicone or an acrylate, which is then placed in the light path of a solid-state LED to produce a white light. This quantum dot method is robust, relatively inexpensive, and it produces light with good color rendering. However, the method is not without its disadvantages. For example, quantum dots can agglomerate when formulated into LED encapsulation films, thereby reducing their optical performance. Furthermore, even if the quantum dots are successfully incorporated into the LED encapsulation film, oxygen can still migrate through the film to the surfaces of the quantum dots, which can lead to photo-oxidation and, as a result, a drop in quantum yield (QY). Finally, current LED encapsulation films are brittle, which makes them difficult to process and handle during film manufacturing.
Thus, there is need in the art for a method to fabricate high quality quantum-dot based films that are both robust and resistant to photo-oxidation.